1. Field of the Invention
The present invention relates generally to signal tracking systems and, more specifically, to a passive system for tracking/locating a transmitter.
2. Background of the Invention
Determining the location and/or tracking a moveable transmitter is useful for many purposes. For instance, there frequently exists an urgent need for locating the source of an emergency 911 telephone call from a mobile phone. Previous options for this application included utilizing GPS location devices but these require additional circuitry, additional antennas, do not necessarily operate indoors, and may be slow to obtain a location fix.
Other tracking requirements might include tracking an astronaut in space during extravehicular activity. Another desirable use might be to track small vehicles in space such as, for instance, a small one-foot diameter autonomous vehicle for flying anywhere around the Space Station and transmitting video from an onboard camera.
It is also increasingly important to be able to locate, track, communicate with or activate vehicle systems wirelessly, and/or otherwise monitor fleets or individual trucks, automobiles, containers, and other moveable targets. Such monitoring is increasingly utilized and has, for instance, become a standard feature on many automobiles. Previous prior art options to perform this function typically include utilizing GPS location devices with the same problems as listed above. It would be very useful to somehow provide such monitoring in a low cost, reliable, high-speed manner that avoids/alleviates the problems associated with presently existing GPS location systems.
The above are only a few specific uses, and it will be understood that a small, inexpensive system that is capable of passively tracking a transmitter by utilizing the transmitter's data modulated signal, and which may operate in a noisy environment with mulitpath signals, may be useful for a myriad of applications and systems.
Tracking objects by receiving signals therefrom is well known. For instance, radar has been utilized for most of the last century to track objects. However radar is nonpassive, requires high power, and also requires a recognizable signature at the target for identification.
Various types of signal processing for estimation of signal parameters have been used as far back as 1795. A more recent signal processing method is that of ESPRIT (Estimation of Signal Parameters using Rotational Invariance Techniques) which is discussed in at least one of the subsequently listed patents. However, the ESPRIT technique is based on relatively complicated mathematics. The inventors believe that using the ESPRIT technique for the purposes of the present invention may require at least four antennas, the generation of complex matrices, the calculation of complex eigenvalues and eigenvectors, and the estimation of noise level. The complexity of these requirements effectively renders the ESPRIT technique unacceptable for many applications requiring low weight and power and small size.
It might also be noted that techniques exist for locating the source of a magnetic field but such techniques require a powerful-magnetic pulse generator at the source.
The following patents show attempts to solve problems related to the present invention but do not show the solution provided by the present invention:
U.S. Pat. No. 5,987,016, issued Nov. 16, 1999, to R. He, discloses a method for tracking a mobile communication signal, which operates in a code division multiple access wireless communication system including an antenna, and a base site receiver having at least two receiver tracking fingers, includes receiving at the antenna a first multipath signal arriving at an on-time pn-offset with an associated advanced pn-offset value and retard pn-offset value and receiving at the antenna a second multipath signal arriving at an on-time pn-offset with an associated advanced pn-offset value and retard pn-offset value. The method further includes determining a spacing between the first multipath signal and the second multipath signal, and adjusting the at least two receiver tracking fingers based on the advanced pn-offset value of one of the multipath signals and the retard pn-offset value of the other multipath signal.
U.S. Pat. No. 5,691,974, issued Nov. 25, 1997, to Zehavi et al., discloses a method and apparatus for tracking the frequency and phase of signals in spread spectrum communication systems that makes more efficient use of available carrier frequency and phase information by utilizing a substantial portion or all of the energy occupying the frequency spectrum of a received carrier signal, including energy from communication signals intended for other system users. Multiple spread spectrum communication signals are input in parallel to data receivers where they are despread using preselected despreading codes at an adjustable phase angle and decoded over multiple orthogonal codes active within the communication system. Multiple decoded signals are then combined to form a single phase detection signal which is used by at least one tracking loop to track frequency and phase of the carrier signal for the received communication signals. The tracking loop generates a timing signal which is used to adjust the phase angle used during despreading. In further embodiments, the communication signals are despread using appropriate PN codes and separated into in-phase (I) and quadrature channels (Q) where data symbols are processed by fast Hadamard transformers to generate corresponding data bits. The data is formed into pairwise products between the channels and summed over multiple or all active subscriber orthogonal codes. This sum indicates a degree to which the estimated phase differs from the actual phase of received communication signals and is used to adjust the phase of application for the PN codes.
U.S. Pat. No. 5,621,752, issued Apr. 15, 1997, to Antonio et al., discloses a system and method for adaptively sectorizing channel resources within a digital cellular communication system is disclosed herein. The system includes an antenna arrangement for providing at least first and second electromagnetic beams for receiving a first information signal transmitted by a specific one of a plurality of users, thereby generating first and second received signals. A first set of beam-forming signals are then generated from the first and second received signals. A demodulating receiver is provided for demodulating at least first and second beam-forming signals included within the first set of beam-forming signals, thereby producing first and second demodulated signals. The system further includes a tracking network for tracking multipath information signals, received from various positions and angles of incidence, based on comparison of the first and second demodulated signals.
U.S. Pat. No. 6,147,641, issued Nov. 14, 2000, to J. Issler, discloses a process for the autonomous reduction of acquisition and tracking thresholds of carriers received in orbit by a receiver accessing an orbital navigator inside or outside said receiver, the latter having at least one phase loop. The phase loop, which is responsible for the acquisition and/or tracking of the carrier, is “pushed” by the fine speed aid and takes up the error between the real speed and the calculated speed. The search for the Doppler frequency of the carrier received takes place around a frequency prediction maintained by the fine speed aid coming from the orbital navigator.
U.S. Pat. No. 5,914,949, issued Jun. 22, 1999, to G. Y. Li, discloses a finger tracking circuit for a rake receiver, a method of tracking a carrier signal and a wireless infrastructure. The finger tracking circuit includes: (1) a timing error subcircuit that determines a timing error in a current power control group (“PCG”) of a carrier signal to be tracked and (2) a feedback subcircuit that applies a gain signal that is a function of a data rate of the carrier signal and a signal-to-noise ratio (“SNR”) to the timing error subcircuit, a convergence rate of the finger tracking circuit therefore depending on the data rate of the carrier signal.
U.S. Pat. No. 5,859,612, issued Jan. 12, 1999, to K. S. Gilhousen, discloses a method for determining the position of a mobile station within a cellular telephone system having a plurality of base stations. A signal is transmitted from a rotating antenna. The rotating antenna has a beam which rotates around a cell in the cellular telephone system. The beam has a rotational timing that is known by the mobile station. The signal is received at the mobile station. Based on a reception time when the signal is received by the mobile station, an angular displacement value corresponding to the position of the mobile station is determined. A first round trip signal propagation time between a stationary antenna and the mobile station is measured using a voice information signal. The position of the mobile station is determined in accordance with the angular displacement value and the first round trip signal propagation time. A method for determining the position of a mobile station within a cellular telephone system having a plurality of base stations. A voice information signal is transmitted from the mobile station. The voice information signal is received with a first antenna having a rotating beam for receiving the signal. Based on a reception time when the voice information signal is received by the first antenna, an angular displacement value corresponding to the position of the mobile station is determined. A first round trip signal propagation time between a second antenna and the mobile station is measured. The position of the mobile station is determined in accordance with the angular displacement value and the first round trip signal propagation time.
U.S. Pat. No. 5,602,833, issued Feb. 11, 1997, to E. Zehavi, discloses a method and apparatus for generating orthogonally encoded communication signals for communication system subscribers using multiple orthogonal functions for each orthogonal communication channel. Digital data symbols for signal recipients are M-ary modulated using at least two n-length orthogonal modulation symbols, which are generally Walsh functions normally used within the communication system. These symbols are provided by a modulation symbol selector typically from one or more code generators, and the modulation is such that M equals a product of a total number of orthogonal functions and the number used to generate individual modulation symbols. Each group of log M encoded data symbols from data processing elements are mapped into one modulation symbol using the modulation symbol selection element according to their binary values. In some embodiments, a fast Hadamard transformer is used for symbol mapping. The resulting communication signals are demodulated by correlating them with the preselected number of orthogonal functions, in parallel, and demodulating the results into M energy values representing each orthogonal modulation symbol. The energy values are mapped into energy metric data using a dual maximum metric generation process. The correlation and demodulation can be accomplished using at least two sets of N correlators, N being the number of functions used, and applying correlated signals to one demodulator for each set of correlators. Each demodulator outputs M energy values representing each of the M mutually orthogonal modulation symbols, which are then combined into a single set of M energy values. In further configurations, coherent demodulators can be used to produce amplitude values for received signals which are then combined with dual maximum metric results to produce composite metric values for data symbols.
U.S. Pat. No. 4,750,147, issued Jun. 7, 1988, to Roy, III, et al., discloses an invention relating generally to the field of signal processing for signal reception and parameter estimation. The invention has many applications such as frequency estimation and filtering, and array data processing, etc. For convenience, only applications of this invention to sensor array processing are described herein. The array processing problem (addressed is that of signal parameter and waveform estimation utilizing data collected by an array of sensors. Unique to this invention is that the sensor array geometry and individual sensor characteristics need not be known. Also, the invention provides substantial advantages in computations and storage over prior methods. However, the sensors must occur in pairs such that the paired elements are identical except for a displacement which is the same for all pairs. These element pairs define two subarrays which are identical except for a fixed known displacement. The signals must also have a particular structure which in direction-of-arrival estimation applications manifests itself in the requirement that the wavefronts impinging on the sensor array be planar. Once the number of signals and their parameters are estimated, the array configuration can be determined and the signals individually extracted. The invention is applicable in the context of array data processing to a number of areas including cellular mobile communications, space antennas, sonobuoys, towed arrays of acoustic sensors, and structural analysis.
The above prior art does not disclose a system that uses a herein disclosed technique in one preferred embodiment for extracting a direction vector from a pair of phase shifts. Moreover, the prior art does not disclose the herein disclosed technique for extracting the phase difference between a transmitter and receiver, which in one preferred embodiment may provide a noncoherent demodulation scheme, and then using the phase difference to detect the location of the transmitter.
Therefore, those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.